KABUL, Afghanistan -- U.S. Sen. John Kerry warned Sunday that already shaky U.S.-Pakistani relations have reached a critical juncture as calls grow in the United States to cut some of the billions of dollars in aid to Islamabad following al-Qaida leader Osama bin Laden's killing.

Kerry, who spoke in Afghanistan before traveling to Pakistan, said sober and serious discussion was needed to resolve the widening rift amid growing suspicion that Pakistan's security forces were complicit in harboring the al-Qaida leader, who was killed May 2 in a raid by U.S. Navy SEALs not far from Islamabad.

For its part, Pakistan is angry that it was not told about the raid in Abbottabad until after it was completed.

Kerry -- chairman of the influential Senate Foreign Relations Committee and the most senior American official to travel to Pakistan since the raid occurred -- sounded a hopeful tone.

"I think for the moment we want to be hopeful and optimistic that we can work our way through this, get over this hiccup, and find a positive path forward," he said.

But he made clear that patience was running thin in Washington after it was discovered that the terror leader had been living for years in Pakistan.